Mechanical gear reducers and increasers are commonly used in industry to provide for proper shaft speed reduction or increase between a prime mover such as an electric motor or other drive means and a conveyer or other output systems. Mechanical reducers provide for increased output torque by mechanically reducing the speed of the driving motor. Conversely, mechanical increasers provide for increased output speed by mechanically increasing the speed of the driving motor. For convenience herein, speed reducer will be referred to, but it should be understood that the principles of the present invention are equally applicable to increasers as well as other systems where it is desirable to mount a gear or the like onto a shaft.
The speed reducer may be driven by a prime mover through a belt drive, direct connection or the like and a multitude of reduction ratios may be utilized. The reducer provides output power to conveyors or the like in a variety of industries including, but not limited to, sand and gravel operations, food processing, feed and grain operations, chemical processing, mining, shipping, or aggregate conveyer systems.
Speed reducers are designed with a varying housing size depending upon end use and application and each housing size may be designed with a multitude of gear ratios. Thus, in order to cover the entire product range a large number of parts are required. Since asset management of speed reducer manufacturers and distributors discourages them from carrying an inventory of completely assembled reducers in every ratio and in sufficient quantity for each ratio, it is desirable to have a means to make an inventory of gear reducers more flexible to customer needs.
One system to create flexible inventory is to design the product with common output gear sets and vary the total ratio by changing the ratio in the input or intermediary stages. This is accomplished by using different sized change gear sets in response to the desired reduction ratio. In this method of inventory management, the issue then becomes the most suitable way to attach the change gear to the pinion shaft.
One method of attaching the change gear to the pinion shaft involves the use of a tapered gear bore fitted onto the matching taper of a pinion. The gear is retained in position on the pinion shaft by a lock nut. Although this method does achieve a flexible inventory, there are several disadvantages associated with it. First, the tapered gear bores and matching tapered pinion shafts are more difficult and costly to machine than traditional straight cylindrical bores. Second, great care must be taken to tighten the lock nut properly to achieve a proper fit of the gear bore to the shaft journal; thus, the integrity of the fit is heavily dependent upon the skills of the individual making the assembly. Furthermore, since tightening of the nut provides the interference fit of the gear to the shaft, the interference fit cannot be relied upon to transmit any of the load. Thus, a key must be designed to carry all of the load.
Another disadvantage to the tapered bore system is that the fit may appear correct at installation, however, if any debris on either the gear bore or pinion journal is present, an incorrect fit will be made, loosening over time will occur, and gear failure will result. Moreover, debris between the shaft and gear bore can cause the gear to be installed less than perfectly square to the shaft resulting in less than desirable gear performance. Thus, while cleanliness is always desirable, it is essential in the tapered bore interference fit for adequate performance.
Another common method utilizes a straight gear bore and a straight shaft. Since an interference fit is desirable for the proper functioning of the reducer, the gear is heated and then shrunk onto the shaft. There are several disadvantages associated with this method. First, special ovens are desirable to heat the gear to approximately 300 degrees Fahrenheit. Second, overheating the gear can result in damage to the gear by changing its metallurgical structure. This frequently occurs when assemblers without ovens try to heat the gear using torches or other localized methods of heating. Also, underheating of the gear may result in the gear becoming only partly assembled to the shaft while initially appearing tight. Eventually, the gear will move after the reducer is placed in service. Finally, heating and shrinking the gear onto the shaft is best done at the factory and in production lot sizes. This method does not lend itself to quick response to customer needs and inventory flexibility.